8 Signal transmission – GROUNDING - CMCommon mode noiseNon perfect grounds often translate into common mode noise problems.CMV couples into a circuit if grounded at more than one point. The coupling can happen via a noise current flowing through a common impedance or by induction of a noise voltage in the ground loop.Some well known mitigation methods are:Single ground systems (float source or receiver)Open ground loop (CM chokes, transformers, optos, isolation amplifiers)Common mode filteringBalanced transmission/differential amplifiersGuarded amplifiers1/73

9 Signal transmission – GROUNDING - CMSingle ground pointZSG is the isolation impedanceIf ZSG is high then Ic2 is strongly reduced.Shielding reduces the capacitive nature of ZSG.Often not possible to float the source.Common mode chokesCM currents generate a non cancelling flux in the choke.In practice, due to physical limitations such as limited permeability and number of turns, common mode chokes provide only moderate attenuation to CM noise.ZSG

10 Signal TRANSMISSION – grounding – CMCM filteringAttenuation of HF common mode at frequencies where the receiver amplifier circuit has limited or no common mode rejection.Passive filters (LC or RC) are commonly used. An example of an RF filter for an instrumentation amplifier is shown below.Guarded amplifiersThe guard shield works in conjunction with a floating receiver and a shielded cable to reduce capacitive coupled common mode noise.Without the guard, CM noise would flow from A back to B through R1 and R2.1/73

11 Signal transmission – GROUNDING - CMDifferential/balanced inputsDifferent types of differential input circuits can be used:Difference amplifierInstrumentation amplifierFully differential amplifierCircuitInput impedancekΩ range – depends on gain resistors, which can’t be too high to limit noiseHigh – corresponds to the input impedance of the buffer amplifierskΩ range – depends on the gain resistors, which can’t be too high to limit noiseCMRDepends on matching between gain resistor ratios !Matched networks often usedHigh, at least in the case of integrated instrumentation amplifiersADC signal conditioningEasy level adapting for ADC inputsWell suited for driving differential ADC inputs and transmission lines. Easy level adapting and anti alias filteringOther-Needs return path for the bias current in case of floating source.

12 Signal transmission – CablingCoaxial vs Shielded twisted pairSTP: preferred below 100kHz. Shield is not a signal conductor.Coaxial: more uniform characteristic impedance, lower losses. Shield is part of signal path, so noise currents should notbe allowed to flow. For high frequencies, skin effect makes it behave like a triax.Where and how should shields be grounded ? The answer depends on:Type of cable (Coaxial or STP)Frequency range of the transmitted signal and noise voltagesNature of the noise coupling (capacitive or magnetic?)Circuit impedances (source and receiver floating or grounded?)

13 Signal transmission – Cabling - shieldingA grounded shield protects against capacitive coupling. If large CMVs are present a shield grounded on both sides will conduct a noise current that can couple with the inner conductors.Copper shields provide no magnetic shielding. The best way to shield against magnetic coupling is to reduce the surface of the signal loop -> twisted pair cables. Use coaxial for frequencies where the signal current returns via the shield and not through ground (f > 5 fshield_cutoff).In low level systems grounded at both ends where magnetic fields are present, the surface of the ground loop (LO to GND) must also be minimized.

14 Signal transmission – Cabling - shieldingShielded twisted pairWhere power frequency common mode voltages are present, and the signal being transmitted is a low level, low frequency voltage signal, the shield should be grounded on one side only (receiver end).If either the source or the load are floating the shield should be grounded at one side only as shown in A and B (except for the case of a guard shield).For all other cases, shields should be grounded on both sides (E).Coaxial cableIf either the source or the load are floating the shield should only be grounded at one side only (C,D).For all other cases, shields should be grounded on both sides (F).

16 Signal transmission – circuit impedanceCurrent transducer output – remote sensing ?OpAmpPrecisionAmplifierBurdenResistorOutputVoltageIsHi SenseHiLo SenseLoRcableOpAmpPrecisionAmplifierBurdenResistorOutputVoltageIsHi SenseHiLo SenseLoDCCT outputs are often available in 4 wire for remote sensing.+ Eliminates error due to voltage drop in the cable- Gain of the differential amplifier becomes dependent of cable impedanceA two wire transmission with a high impedance differential input at the receiver end gives good results. The differential input provides the required CMR.

18 Signal ConditioningThe functions to be performed by the signal conditioning circuits derive from the nature of both the signal and the receiver and may comprise:Current to voltage conversion (not covered here)Filtering: CM and series (discussed in previous section)Multiplexing/switchingBuffering/ impedance adaptingDifferential inputLevel adaptationAnti Alias filtering1/73

20 Signal Conditioning Buffering/ impedance adapting Level adaptingMultiplexing/switchingUse high impedance inputs to eliminate errors due to mux’s ON resistance.Cross talk and settling time might occur due to source impedance combined with mux’s stray capacitance.Low source impedance also minimizes effect of charge injection from the multiplexer.Buffering/ impedance adaptingZSource and Zreceiver form voltage divider. Buffering ensures Zreceiver is large, maximizing the voltage signal at the receiver input.Buffers are used in combination with differential amplifiers to create balanced inputs.Unity gain amplifiers are sensitive to capacitive loads – particular important if dynamics is an issueLevel adaptingAttenuation or amplification of a voltage signal using voltage dividers and op amp circuits.Level shifting, in particular for ADCs with differential inputs.On fully diff amplifiers the Vocm pin allows the output CMV to be adjusted for precision level shifting.-10V..10Vsignal2.5V ± 1V1/73

21 Anti-alias filtering/ sampling strategiesThe anti-alias requirements/strategy depend on the sampling strategy:Nyquist-Shannon sampling: fsampling > 2.fmax.signalThe anti-alias filter must provide appropriate attenuation above Fmax.Cutoff frequency and filter order depend on desired dynamic range.Below we can see the effect of aliasing on dynamic range. On the right we see the response of a 10th order anti alias filter designed to achieve 60dB dynamic of range for a 3kHz signal bandwidth and 12kSPS sampling speed.alias freealias limitsdynamic range1/73

22 fsampling >>> fNyquistAnti-alias filtering/ sampling strategiesOversampling and decimationfsampling >>> fNyquistInput analogue anti-alias filter significantly relaxed. The filter roll off needs to guarantee the dynamic range for (k.fs)/2 instead of fs/2.Signal is subsequently digitally filtered and decimated down to the band of interest.Digital low pass has to provide anti alias for fs/2 to guarantee the decimation process is alias free.Synchronized samplingIn PC applications with well known ripple noise, such as PWM converters, aliasing can be used to achieve ripple elimination.In this case, Shannon’s theorem is not respected but used for our advantage.If sampling and switching are perfectly synchronised, the effect of aliasing will be the reconstruction of the average value of the sampled signal, eliminating the ripple.ReconstructedsignalLoad current(Sampled signal)Load voltageTs1/73

23 anti alias filtersOne pole passive filters are still used where impedance does not impact the conversion process like at input of DS converters. Otherwise active filters are preferred as they provide isolation and low output impedance.A commonly used circuit is the non inverting second order Sallen-Key filter. Another popular circuit is the inverting double pole multiple feedback shown below. Cascading several stages allows higher order filtering.Double pole multiple feedback1/73

24 Precision components – voltage referencesMain technologies:Bandgap: Temperature compensated. Low cost, medium accuracy applications.Buried Zener: Very good long-term stability and low noise. High accuracy applications, higher cost.Both types can include additional on-chip circuitry to further minimize temperature drift.Important specification parametersInitial error: importance of this parameter depends on calibration strategyTemperature coefficient: auxiliary circuits might be included in the reference for better TCThermal hysteresis: change in output voltage after temperature cycling. Function of packaging, IC layout. Can often be improved by a burn in process.Noise: Includes broadband thermal noise and 1/f noise.Long term drift: can be improved by a burn in process which normally involves several days power cycling at Tambient>80ºC.Line and load regulation1/73

26 Precision components - ResistorsNetwork resistorsFor voltage division or amplification, precise ratio devices are now readily found. TCR tracking is of most importance for resistors used as ratio devicesMetal foil reaches best accuracies, followed by thin filmTolerance ≠ Precision (a 0.05% thin film will eventually drift to 1% and prove worse than a 0.5% metal foil which has much better stability).Power coefficient – change due to self heating (TC: changes due to ambient temperature). In an amplifier configuration with gain > 1 the power PR2 > PR1 which means gain resistor internal heating will be different. Minimizing absolute TC (linked to PC) is therefore also an important factor.Load life stability – mechanical effect of stress relaxation of the resistive element’s internal construction, normally hundreds/thousands of hours.Thermal and current noise1/73

29 Analogue to digital convertersChoice of ADC architectureCriteria: precision, resolution, dynamic range, speedSuccessive Approximation Register (SAR) converters typically range from 8 to 18 bits with sample speeds up to several MSPS. They have the ability to be connected to multiplexed inputs at a high data acquisition rate.Delta-sigma converters (ΔΣ) have virtually replaced the integrating-type ADCs (e.g. dual-slope) for applications requiring high resolution (16 bits to 24 bits) and low speed. They are inherently linear and monotonic.1/73

30 Analogue to digital convertersDelta Sigma – Oversampling, noise shaping, digital filtering and decimationFigure A shows the noise spectrum for a “Nyquist” ADC sampling at fs. Figure B shows how oversampling at a K.fs (k = oversampling ratio) spreads the noise energy over a wider frequency range. Figure C shows the effect of the DS integrator in shaping the noise. This shaping can be exploited to remove most of the noise using a digital filter.The resolution that can be obtained with a DS depends on the oversampling ratio, noise shaping and the digital filter. On designing the digital filter, a tradeoff between bandwidth and resolution has to be done.Because of oversampling and latency, sigma-delta converters are not often used in multiplexed signal applications.Idle tones can be a problem in DS ADCs: Tones are caused whenever the modulator output sequence falls into a cyclic mode. They depend on the modulator (dc) input signal and the initial conditions of the integrator outputs.1/73

32 Temperature coefficientTemperature related errors (TC, thermal voltages) can be minimized by:Using compensation algorithmsFirst order vs second order – the TC can vary with temperature so a linear compensation might not be enoughIndividual vs standard TCsIt might not be possible to use individual TC values for the elements to be compensated, so an average TC might be used as long as the TC spread is not too important.1/73